GB2408421A - Method for selecting a channel coding scheme and apparatus therefor - Google Patents

Abstract

The present invention relates to the selection of a channel coding scheme for communications in a wireless communication system. In the described arrangement, radio condition information for idle subscriber devices is collected and a channel coding scheme is selected from a plurality of available channel coding schemes on the basis of the collected radio condition information, and stored. The stored selected channel coding scheme can then be used on initiation of a communication with the subscriber device. The present invention relates to a method, apparatus and associated storage medium.

Description

240842 1

METHOD FOR SELECTING A CHANNEL CODING SCHEME AND

APPARATUS THEREFOR

The present invention relates to the selection of a channel coding scheme for communications in a wireless communication system.

Many different wireless communication systems are known. In a cellular wireless communication system, the total area over which communication services are provided is divided into a number of cells. Each cell has a base station supporting wireless communication links with subscriber communication devices within the cell. Communications in the direction from the base station to the subscriber device are called downlink or forward communications, and communication in the direction from the subscriber device to the base station are called uplink or reverse communications.

Channel coding is generally applied to data being transmitted on both uplink and downlink channels in order to provide protection against errors in the received data caused by radio transmission errors.

In some communication systems, different channel coding schemes, that provide different levels of protection to the channel data being transmitted, are provided.

In situations where the radio propagation conditions are good, less channel coding protection is needed to ensure correct transmission of the channel data than in situations where radio propagation conditions are bad.

Typically in systems that provide different channel coding schemes, at the start of a data transfer a predetermined initial channel coding scheme is used. Thereafter during the data transfer the channel coding scheme will be adjusted to accommodate the radio propagation conditions affecting the wireless communication. Thus for example the channel coding scheme will be adjusted from the predetermined initial channel coding scheme to a channel coding scheme with a low level of protection for the channel data for a subscriber device in good radio conditions. In less favorable radio conditions, for example when the subscriber device is behind an obstruction such as a building, the channel coding scheme will be adjusted from the predetermined initial channel coding scheme to a channel coding scheme with a higher level of protection for the channel data.

Typically the time necessary to determine the correct level of channel coding needed for a particular communication with a subscriber device is long compared with the time needed for the communication, so that a suboptimum level of channel coding is used during all or a substantial part of the communication. The use of sub-optimum levels of channel coding creates a number of problems.

Firstly, the pre-determined channel coding scheme may be too conservative for a subscriber device in good radio conditions, leading to a limitation in throughput for the subscriber device when transmitting data, for example in a packet data system. In this respect it should be noted that the effective channel data throughput in higher channel coding schemes may be twice or more as high as the effective data throughput using lower channel coding schemes.

Secondly, the predetermined channel coding scheme may not be robust enough for a subscriber device in poor radio conditions. In this situation, many of the initial data blocks will have to be re-transmitted because the originally transmitted data blocks were received with too many errors, and the re-transmitted data blocks may also not be received since the same channel coding scheme may also be used for the re- transmitted blocks.

This re-transmission of data blocks in turn has a number of undesirable consequences for the subscriber device and for the communication system as a whole. In particular, there will be a high transmission delay of current and buffered data for the subscriber device, leading to poor end- to-end user throughput of data, and perception of poor quality in time- dependent applications such as speech. In addition, since the communication is taking longer than necessary because of the re- transmission, higher queuing delays will be experienced with respect to communications with other subscriber devices in the same cell. Furthermore the re-transmission of the data blocks causes increased interference in neighbor cells which in turn will lead to the use of increased transmission powers in the neighbor cells, which in turn will lead to increased interference in the original cell and in the communication network as a whole.

The present invention seeks to at least partially alleviate the disadvantages of the

prior art.

According to a first aspect of the invention there is provided a method of selecting a channel coding scheme as claimed in claim 1.

According to a second aspect of the present invention there is provided an apparatus as claimed in claim 12.

For a better understanding of the present invention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 shows a schematic diagram of a GPRS communication system in which the invention may be implemented; Figure 2 is a flow diagram illustrating an embodiment Figure 3 is a schematic diagram illustrating the obtaining of radio condition information in accordance with the illustrated embodiment; Figure 4 is a flow diagram showing a method of calculating a channel coding scheme based on radio conditions; Figure 5 illustrates data transfer process in the GPRS communication system shown in Figure 1.

An embodiment will now be described with reference to and in the context of the widely known General Packet Radio Service (GPRS) cellular wireless communication system that is based on the Global System for Mobile communication (GSM) system and provides packet data capability. However, it should be noted that the invention is not intended to be limited to the GPRS system and can be applied to any communication system in which multiple channel coding schemes can be used.

Figure 1 shows a schematic diagram of a GPRS communication system 100 in which the invention may be implemented.

A wireless subscriber device MS 10 is located within a cell 15 of a base station BSS 20 of the communication system 100. As mentioned above, the base station BSS 20 provides communication services to subscriber units located within the cell 15: only one subscriber device MS 10 is shown, for clarity, but it will be apparent that typically the base station can provide communication services for a large number of subscriber units located within the cell 15.

The base station BSS 20 has three main components. A base transceiver station BTS 25 exchanges signaling and traffic data with the MS 10 using logical and physical channels available to the BTS 25. The arrangement and implementation of these channels will be known by a skilled person, and will therefore not be described herein in further detail. A base station controller BSC 30 controls the operation of the BTS 25. Again the function and operation of the BSC 30 will be known to a skilled person, and so will not be described in further detail. A packet control unit PCU 35 is also provided, which handles the transfer of packet data to and from the MS 10 via the BTS 25. Again the general operation and function of the PCU will be known to a person skilled in the art and so will not be described further except in so far as it relates to the invention.

In particular, the PCU 35 has a processor 350 that implements the method described below with reference to Figure 2. In addition, the PCU 35 is provided with a memory 351 for storing context information for idle subscriber units within cell 15, as will be explained below.

In the exemplary embodiment, as will be explained below, the PCU processor 350 maintains in memory 351 idle subscriber unit context information, preferably for all idle mode subscriber units associated with the packet control unit. The idle subscriber unit context information may be implemented in a database in memory 351, for example.

The PCU processor 350 may be a dedicated processor for implementing the method of the invention, but it is envisaged that more generally PCU processor 350 will be implemented as a part of a general PCU processor implementing PCU functions. Thus for example, the method of the described embodiment described below may be embodied in a software program or as a module of a software program running on a PCU processor, as will be understood by a skilled person.

In addition the memory 351 may be implemented as a dedicated memory, or may be implemented as part of a bigger memory, for example a general PCU memory. The memory 351 may be implemented in any form of re-writable memory, such as RAM.

The base station 20 is connected with other elements of the GPRS communication system. Specifically, the base station is coupled to the core GPRS network elements, the Serving Gateway Support Node SGSN 40 and the GPRS Gateway Support Node GGSN 45, that provide switching and routing functions for packets within the illustrated GPRS communication system. The GGSN 45 in turn is coupled to an external packet data network, for example the Internet.

Again, the functions and operation of the SGSN 40 and the GGSN 45 in a GPRS network will be known to a skilled person and are not relevant to the present invention, and therefore will not be explained in further detail.

As will be apparent to a skilled person, a typical GPRS system will include other elements that are not shown herein for clarity.

A flow chart incorporating a method in accordance with an embodiment is shown in Figure 2. As indicated above, the method shown in Figure 2 may be carried out by the PCU processor 350.

In a first step s1 radio condition information relating to an idle subscriber unit, i.e. a subscriber unit that is not currently engaged in data transfer, is obtained. Such radio condition information may be obtained in a number of ways. An exemplary method of obtaining radio condition information relating to a subscriber unit will be explained below with reference to Figure 3.

Next, in step s2, the channel coding scheme appropriate to the radio conditions being experienced by the idle subscriber unit is determined, and the resulting channel coding scheme is stored in part of memory 351 allocated to the idle subscriber unit. Again there are a number of different methods of determining an appropriate channel coding scheme, and an exemplary example of such a method will be explained in more detail with reference to Figure 4.

These two steps are periodically repeated while the subscriber unit remains in the idle condition. Thus, in step s3 it is determined whether any communication with, i.e. any data transfer to or from the subscriber unit, is required, or whether the subscriber unit is still in idle mode. This determination can be made for transmissions to the subscriber unit by detecting receipt of data packets addressed to the subscriber unit by the packet control unit, and by the receipt of a Temporary Block Flow (TBF) request from the subscriber unit for transmissions from the subscriber device. While no transmissions are required and so the subscriber unit remains in idle mode, step s3-n, steps s1 and s2 are periodically repeated.

In this way, an accurate and up-to-date record of the optimum initial channel coding scheme to be used for an idle mode subscriber unit is maintained, the selection of the optimum initial channel coding scheme being based on the prevailing radio propagation conditions.

Thus, once a data transfer is required for an idle subscriber unit, step s3-y, the initial channel coding scheme for that subscriber unit can be obtained from the memory 351 and used to set up the data transfer, step s4. Once the data transfer is in progress, the channel coding scheme can be modified depending upon the radio information reported during the data transfer, step s5. Figure 5 illustrates in more detail an exemplary data transfer process in accordance with steps s4 and s5 in more detail.

While the data transfer is ongoing, step s6-n, the channel coding scheme is modified in accordance with the changing radio conditions, as set out above in step s5. Once the data transfer ends, step s6-y, the subscriber unit returns to the idle mode and to step s1.

Optionally, the last channel coding scheme used during the data transfer may be stored in the memory 351, for use as the initial channel coding scheme for a data transfer set up prior to the next idle mode update of the channel coding scheme in view of the radio conditions experienced by the idle mode subscriber unit in accordance with steps s1-s3 outlined above. This will enable the subscriber unit to be allocated the most appropriate initial channel coding scheme for a subsequent data transfer.

An exemplary method of obtaining radio condition information relating to an idle subscriber unit will now be described with reference to Figure 3. In this exemplary method described with reference to and in the context of a GPRS system, existing GPRS system messages are used to obtain radio condition information from idle mode subscriber units. This means that it is not necessary to modify existing systems to implement the inventive concept described herein.

However, the invention is not intended to be limited to the use of such messages, and any suitable existing, new or custom messages appropriate to the communication system may be used.

The exemplary method of Figure 3 utilises a broadcast method of communicating with one or more idle mode subscriber units. A broadcast method enables efficient communication with a number of idle mode subscriber units. However, it would also be possible for a targeted request for a radio condition measurement update to be send to a targeted idle mode subscriber unit or a small group of idle mode subscriber units.

Firstly, the PCU controller 350 sends a message requesting an idle mode subscriber unit to provide one or more radio condition measurement updates.

The request may specify whether more than one radio condition measurement update is required, and may also specify a frequency at which measurements are to be made or a time interval between measurements.

In the exemplary messaging in a GPRS system, shown in Figure 3, the PCU controller 350 orders idle mode subscriber units to provide EXTENDED MEASUREMENT REPORTS containing the radio condition information. This is achieved by the broadcast of a PS15 message in which the EXT_MEASUREMENT_ORDER parameter is set to EMI, as defined in the GPRS specifications GSM04.60 step s7. The parameter EXT_REPORTING PERIOD in the PS15 message may be set to define the frequency at which the idle mode subscriber unit is to send the extended measurement reports to the PCU controller 350.

In response to receipt of such a message, the idle mode subscriber unit performs radio condition measurements, and sends a radio condition report message, containing the radio condition measurements, to the PCU controller 350, step s8.

The idle mode subscriber unit may measure at least the received signal level and an interference level, and may report these measurements to the PCU controller In the exemplary embodiment, the idle mode subscriber unit measures at least the received signal level, RXLEV, and the interference level l_LEVEL as specified in the GPRS specification GSM 05.08 and reports these radio condition measurements to the PCU controller 350 in a PACKET MEASUREMENT REPORT.

If the EXT_REPORTING PERIOD parameter is set, the idle mode subscriber unit repeats the measurement of at least the received signal level, RXLEV, and the interference level l_LEVEL as specified in the GPRS specification [05.08 standards reference] and reports these radio condition measurements to the PCU controller 350 in a PACKET MEASUREMENT REPORT step s9.

In response to the receipt of the radio condition report message (PACKET MEASUREMENT REPORT in the above exemplary embodiment) the PCU controller 350 creates a new "packet idle subscriber unit context" area in the memory 351 for the subscriber unit, if such a context area has not already been created. The PCU controller 350 then processes the radio condition information and updates the channel coding scheme field in the respective context area, as described with reference to step s2 of Figure 2 and with more detail with reference to Figure 4.

An exemplary method of determining the channel coding scheme for an idle mode subscriber unit based on radio condition information for the subscriber unit is set out in Figure 4. In this embodiment, four channel coding schemes CS1- CS4 are available for selection: however, the invention is not intended to be restricted to the use of four channel coding schemes. Furthermore, in this embodiment the radio condition information considered in determining the channel coding scheme is the received signal level RXLEV and the interference level l_LEVEL as described above with reference to Figure 3. However, in fact any single or multiple radio condition measurements could be used as will be appreciated by a skilled person. It is not necessary in all embodiments to use both received signal level and interference level in determining the channel coding scheme.

In step s10 the received signal level RXLEV and the interference level l_LEVEL for an idle mode subscriber unit, as described above with reference to Figure 3, are received.

Initially, in step s11, it is determined whether the interference level l_LEVELis less than an interference threshold, Th_l. If the interference level is greater than the interference threshold Th_l, s1 1-n, the channel coding with the greatest protection for the data, CS1, is allocated in step s12.

If the interference level is less than the interference threshold Th-l, s1 1-y, it is determined whether the received signal level RXLEVis less than a first received signal level threshold Th_a, step s13. If the received signal level RXLEVis less than a first received signal level threshold Th_a, s1 3-y, again the channel coding scheme with the greatest protection for the data, CS1, is allocated in step s12.

If, however, the received signal level RXLEVis greater than a first received signal level threshold Th_a, s1 3-n, it is determined whether the received signal level RXLEVis less than a second received signal level threshold Th_b, step s14. If the received signal level RXLEV is less than the second received signal level threshold Th_b, s14-y, a channel coding scheme with less protection for the data, CS2, is allocated in step s15.

If, however, the received signal level RXLEV is greater than the second received signal level threshold Th_b, s1 4-n, it is determined whether the received signal level R)(LEV is less than a third received signal level threshold Th_c, step s16. If the received signal level R)(LEV is less than the third received signal level threshold Th_c, s1 6-y, a channel coding scheme with even less protection for the data, CS3, is allocated in step s17. If the received signal level R)(LEV is more than the third received signal level threshold Th_c, s1 6-n, a channel coding scheme with the least protection for the data, CS4, is allocated in step s18.

Thereafter, after the appropriate channel coding scheme has been determined in each of steps s12, s15, s16, s18, the determined channel coding scheme is stored in step s19. As indicated above with reference to Figures 2 and 3, the PCU controller 350 may store the determined channel coding scheme in a packet idle subscriber unit context area for the subscriber unit in the memory 351.

Figure 5 illustrates in outline the alteration of the channel coding scheme used during a communication in accordance with an embodiment. Figure 5 relates to a situation in which the PCU 35 receives incoming packet data destined for an idle mode subscriber unit 10 associated with the PCU 35 i.e. a subscriber unit 10 within the cell 15 of the BSS 20. However, the skilled person will understand that similar processes apply to communications initiated by the subscriber unit, and also to circuit switched calls capable of utilizing multiple channel coding schemes.

Firstly, the PCU 35 receives a packet transfer request notification, s20, from the core network, for example the SGSN 40, indicating packets to be sent to the subscriber unit 10. The PCU controller 350 retrieves the stored channel coding scheme from the packet idle subscriber unit context area for the subscriber unit in the memory 351, an assigns this as the initial channel coding scheme for the communication, as described above with reference to Figure 2. The PCU controller then informs the subscriber unit of the assigned channel coding scheme during set up signaling for the packet transfer shown in step s21 in Figure 5. Further details of the set-up signaling s21 will be familiar to a skilled person and so will not be described in more detail.

Once the set up signaling s21 is complete, the data transfer is carried out s22.

Initially, the channel coding scheme used will be the assigned channel coding scheme which was stored for the subscriber unit. Thereafter during the data transfer, as described above in connection with step s5 of Figure 2, the channel coding scheme may be modified in view of radio condition information reported during the communication.

Once the data transfer s22 has been completed, end of data transfer or teardown signaling s23 may be used prior to the end of the communication.

Although it is assumed in the description of the exemplary embodiment above that the memory 351 has sufficient capacity to store idle subscriber unit context information for all associated idle subscriber units, in a particular implementation a limitation on the number of idle subscriber unit contexts may be imposed. In this situation it is envisaged that the PCU controller 350 may reduce the number of idle mode subscribers reporting prevailing radio conditions.

One way this could be achieved would be to enable the PCU controller 350 to remove or fail to create or renew idle subscriber unit contexts for subscriber units least likely to require a communication. Examples of such subscriber units are subscriber units that have been in a standby mode for the longest time: and then subscriber units that have been in a ready state and packet idle mode for the longest period. One way this can be achieved in accordance with the exemplary embodiment described above in the context of a GPRS system is for the PCU controller 350 to send a PACKET MEASUREMENT ORDER with the EXT_MEASUREMENT_ORDER set to RESET to the selected subscriber units until the load on the memory 351 reduces to an acceptable level.

Thus the present invention provides a method and an apparatus for assigning the most suitable channel coding scheme to be used for a communication with a subscriber unit at the initiation of the communication. This is achieved by selecting a channel coding scheme from a plurality of channel coding schemes based on radio condition information for the subscriber device prior to initiation of the communication. In a described embodiment, this is achieved by requesting an idle mode subscriber unit to provide radio condition information.

Thus the invention enables the most suitable channel coding scheme to be assigned at the start of a communication, for example a temporary block flow in a GPRS system. The use of the invention may tend to provide one or more of the following advantages: Higher end-to-end user throughput Lower user latency Lower overall interference in the communication system Increased system capacity Increased average revenue per user (ARPU) Reduced operational expenditure (OPEX) and capital expenditure (CAPEX) by system operators Lower queuing delays therefore less buffer size required at network elements such as PCU and SGSN

Claims (17)

1. A method of selecting a channel coding scheme for use at initiation of a communication with a subscriber unit, from a plurality of available channel coding schemes, wherein the selection is made based on radio condition information relating to the subscriber unit.

2. The method as claimed in claim 1 wherein the selection is on the basis of the most recently available radio condition history.

3. The method as claimed in any preceding claim wherein the selection is made based on radio condition information for the subscriber unit in an idle mode preceding the communication.

4. The method as claimed in any preceding claim wherein the channel coding scheme used for a preceding communication is selected for use at initiation of a communication with the subscriber unit.

5. The method as claimed in any preceding claim further comprising the step of requesting an idle mode subscriber unit to provide radio condition information.

6. The method as claimed in claim 5 wherein the step of requesting an idle mode subscriber unit to provide radio condition information comprises the step of sending a PACKET MEASUREMENT ORDER message as defined in GPRS standards.

7. The method as claimed in any preceding claim further comprising the steps of selecting a channel coding scheme based on the reported radio condition information, and storing the determined channel coding scheme information, wherein the channel coding scheme identified by the stored channel coding information is selected for initiating the communication with the subscriber unit.

8. The method as claimed in any preceding claim wherein the radio condition information includes information relating to interference levels experienced by the subscriber unit, and the most robust channel coding scheme is selected when the interference level is worse than a predetermined threshold.

9. The method as claimed in any preceding claim wherein the radio condition information includes information relating to the received signal levels at the subscriber unit, comprising the step of comparing the received signal level with at least one threshold and the step of selecting a channel coding scheme from the plurality of channel coding schemes on the basis of the result of the comparison.

10.The method as claimed in any preceding claim wherein the radio condition information comprises information relating to interference levels experienced by the subscriber unit and/or information relating to the received signal level of signals received by the subscriber unit from a serving base station.

11. A storage medium containing processor-implementable instructions for controlling a processor to carry out the method as claimed in any preceding claim.

12.Apparatus comprising: a processor for selecting a channel coding scheme, for use at initiation of a communication with the subscriber unit, from a plurality of channel coding schemes based on radio conditions of a subscriber unit.

13. The apparatus as claimed in claim 12 further comprising a memory for storing the selected channel coding scheme.

14.The apparatus as claimed in claim 13 wherein the processor initiates a communication with the subscriber unit using the stored channel coding scheme.

15. The apparatus as claimed in one of claims 12 -14 wherein the apparatus is a packet control unit

16.A method substantially as herein described with reference to the accompanying drawings.

17.Apparatus substantially as herein described with reference to the accompanying drawings.